The present invention relates to an electron gun for use in a television camera tube, and more particularly to a diode type electron gun for generating a laminar flow electron beam.
Such an electron gun has been proposed in a Japanese Patent Application Kokai (Laid-open) No. 39869/75 (corresponding to U.S. Pat. No. 3,894,261). The electron gun includes, as shown in FIG. 1, a thermionic cathode 21, a heater 22, a grid 23, an anode 24, and a disk 25 which is electrically connected with the grid 23 and has an aperture 26. In the above-mentioned electron gun, the grid 23 (including the disk 25) and the anode 24 are applied with positive potentials relative to a cathode potential. Such an electron gun is called the diode type electron gun, and performs a diode operation. Specifically, in this electron gun, no electron lens is formed between the thermionic cathode 21 and the grid 23 so that an electron beam emitted from the thermionic cathode 21 does not form any crossover, and the diameter of the electron beam is restricted by the aperture 26 made in the grid 23 to generate an electron beam 20 having a small diameter. Further, an end of the aperture 26 which faces the thermionic cathode 21, is smaller in diameter than the other end, as shown in FIG. 2. That is, the aperture 26 has, in section, the form of a knife edge in order to prevent the diameter of the electron beam from being increased due to electrons scattered by the side wall of the aperture.
However, it is very difficult to make an aperture having such an ideal form as shown in FIG. 2. Further, since the electron beam emitted from the thermionic cathode includes electrons each of which has a velocity component perpendicular to the axis of electron gun, that is, parallel to a radial direction, it is impossible to reduce in a large degree the number of electrons scattered by the side wall of the aperture. Accordingly, in a television camera tube provided with an electron gun having such a structure as shown in FIG. 1, the spot size of the electron beam is increased due to the above-mentioned scattered electrons, especially when the electron beam is deflected to a corner on the scan area, and therefore the resolution at the corner on the scan area is considerably deteriorated.
Further, a Japanese Patent Application Kokai (Laid-open) No. 129871/79 (corresponding to British Patent Application Laid-open No. GB 2015817A and corresponding to U.S. application Ser. No. 877,080 filed on Feb. 13, 1978) has proposed an electron gun which is an application of the above-mentioned diode type electron gun, and in which an electron focusing lens having substantially no effect on the electron emission at a cathode is formed between a grid and an anode in order that an electron beam slightly converges to form a crossover. This electron gun includes, as shown in FIG. 3, a grid 23 (namely, a first anode) having a first aperture 26, and an anode 24 (namely, a second anode) having a second aperture 28. The anode 24 is partially closed with a disk 27 having the second aperture 28. The diameter of the first aperture 26 is at least twice as large as that of the second aperture 28, and the diameter of the first aperture 26 is made small appropriately. In this electron gun, the grid 23 is applied with a positive voltage of 10 to tens of volts and the anode 24 is applied with a positive voltage which is at least ten times as large as the voltage applied to the grid, that is, a positive voltage of at least 100 volts in order to form between the grid 23 and anode 24 a lens field which has substantially no effect on the electron emission at a cathode 21. Thus, an electron beam having passed through the first aperture 26 forms a crossover, and then the utilizing beam current and the divergent beam angle are controlled by the second aperture 28 having a diameter of about 0.05 mm. That is, an electron beam 20 having a small diameter is generated. In the above-mentioned electron gun, owing to the presence of the crossover, a group of electrons contained in the electron beam have an energy distribution which is for wider than an energy distribution determined by a cathode temperature, and therefore the capacitive signal lag is large.